The present invention relates to a vehicular automatic transmission (including a continuously variable transmission) which is constructed to perform an automatic control for establishing speed change ratios in response to changes in the driving condition while the driver manipulates the shift lever to select drive ranges.
A vehicular automatic transmission comprises a speed change mechanism which includes a plurality of mechanical power transmission paths and is connected to an engine through, for example, a torque converter. Selection of or switching to an appropriate power transmission path, i.e., an appropriate speed change ratio, is automatically performed on the basis of, for example, the accelerator opening and the vehicle speed. Generally, a vehicle with an automatic transmission is equipped with a shift lever, which is operated by a driver. When the driver manipulates the shift lever, a range of speed change ratios or a drive range (for example, a reverse drive range, a neutral range, a forward drive range) is selected in response to the manipulation of the shift lever, and the speed change ratio is automatically adjusted or controlled within the selected drive range (usually in a forward drive range).
When a vehicle equipped with such an automatic transmission is stopped while the transmission is set in the forward drive range, a so-called creeping phenomenon occurs because the driving force from the engine which is idling is transmitted to the transmission through the torque converter and then to the wheels. This creeping phenomenon is beneficial for smooth starting under a certain condition, for example, when the vehicle stops and then starts on an upward slope, but not welcome when the vehicle should be kept stationary. At present, to keep the vehicle stationary, the brake must be operated against the creeping force, which is generated by the engine, so this brake operation reduces the mileage allowance or the fuel efficiency of the vehicle.
To solve this problem, a method to improve the fuel efficiency of such an automatic transmission is proposed. In this method, while the vehicle is kept stationary with the brake pedal being stepped or pressed for brake operation and the accelerator being substantially closed, the transmission is set into a neutral condition although the selection of the drive range is kept unchanged (for example, Japanese Laid-Open Patent Publication Nos. H11-193866, H11-230329, etc.).
In such a transmission which is set in a neutral condition while the brake pedal is pressed for keeping the vehicle stationary, the brake operation is monitored by a brake switch which is turned on while the brake pedal is pressed by the driver. More specifically, the brake switch turns on at the beginning of the brake actuation. However, the response of the brake switch is somewhat delayed when the brake pedal is released because a certain distance in the operational stroke of the brake pedal, which distance depends on how deep the pedal is pressed, must be cleared before the brake switch is actually turned off.
This characteristic property of the brake switch presents a following problem. After the vehicle has come into a halt with the brake pedal being pressed while the transmission is set in a drive range, if the brake pedal is released gradually or the brake pedal is released and kept half way in the operational stroke, then there is a possibility that while the brake operation itself terminates, the brake switch is kept being turned on. If this condition occurs, though the transmission itself is in a neutral, the control system detects that the brake is still being actuated. This condition results in a delayed response in the starting control of the vehicle.
It is an object of the present invention to provide a control system for a vehicular automatic transmission, which system can establish a neutral condition for the transmission set in a forward drive range while the vehicle is stationary with the brake being operated, and which system thereafter enables the vehicle to start smoothly without any control delay when the brake pedal is released for starting the vehicle.
To realize the above objective, the present invention provides a control system (for example, the control valve CV and the electronic control unit ECU described as a preferred embodiment in the following section) for a vehicular automatic transmission (for example, the automatic transmission TM described in the following section, and also a continuously variable transmission). The control system according to the present invention performs an automatic control for selecting and establishing a speed ratio (speed change gear ratio) in response to the condition of the vehicle set in a drive range, and the control system creates a neutral condition if the vehicle is stationary with the brake of the vehicle being actuated and the accelerator for the engine being turned off (i.e., the accelerator pedal is released to turn off the accelerator, but the engine is kept idling) while the transmission is set in the drive range. This control system further comprises a brake operation detector (for example, the brake switch 36 described in the following section), which detects the operational condition of the brake of the vehicle, and an output rotation detector (for example, the output rotation sensor 40 described in the following section), which detects the rotational speed of the output member of the transmission. With this arrangement, the control system terminates the neutral condition and sets a predetermined speed ratio (any of the speed ratios in the drive range) if the rotation of the output member is detected by the output rotation detector, even though the operation of the brake is still detected by the brake operation detector.
According to the present invention, if the operation of the brake is terminated while the transmission is in the neutral condition, then the termination of the actual brake operation is detected accurately by the output rotation detector. For example, when the vehicle is on a bumpy road or on a sloped road, the wheels of the vehicle tend to rotate at least a little. This minute rotation of the wheels can be detected by the output rotation detector, and the control system can control the transmission accordingly and appropriately. In a case where the brake pedal is released gradually or the brake pedal is released and kept half way in the operational stroke (in this case, even though the brake operation detector, i.e., the brake switch, has not detected the release of the brake pedal), the actual termination of the brake operation is detected by the output rotation detector, so the control system immediately shifts the transmission from the neutral condition to a predetermined speed ratio (for example, the LOW speed ratio) in the forward drive range. As a result, there is little delay in the control that terminates the neutral condition and starts the vehicle smoothly.
When the vehicle is on a road which descends forwardly, no control delay is experienced. Therefore, if the control system detects that the vehicle is on a descending road by an inclination sensor (for example, the inclination sensor 37 described in the following section), which detects the inclination of the vehicle in the traveling direction, the control system may maintain the neutral condition even though the output rotation detector detects that the output member is rotating, and the control system may terminate the neutral condition when the brake operation detector detects that the brake is released. In this way, the fuel efficiency of the vehicle is improved by keeping the neutral condition a little longer.
On the other hand, when the vehicle is on an ascending road, the control system should expect a delay in the control for starting the vehicle. Therefore, if the control system detects by the inclination sensor that the vehicle is on an ascending road whose inclination is equal to or greater than a predetermined inclination, then it is preferable that the control system will not allow the transmission to be set in the neutral condition even though the vehicle whose transmission set in the drive range is stationary with the brake being operated and the accelerator being turned off.
For the same reason as described above, the control system may further comprise a first acceleration detector (for example, an arrangement which calculates a first acceleration by the output rotation sensor 40 and the electronic control unit ECU described in the following section), which detects the vehicle""s acceleration in the traveling direction on a basis of changes in the speed of the vehicle, a second acceleration detector (for example, an arrangement which calculates a second acceleration by the G sensor 38 and the electronic control unit ECU described in the following section), which detects the vehicle""s acceleration in the traveling direction based on a value detected by a G sensor (a sensor which detects directly accelerations acting on the vehicle, such as gravitational acceleration and accelerations at the time of the vehicle""s starting and stopping), and an inclination detector (for example, an arrangement which comprises the steps Step S33 and Step S34 described in the following section), which calculates the vehicle""s inclination in the traveling direction from the acceleration values detected by the first and second acceleration detectors. With this arrangement, if the vehicle""s inclination in the traveling direction detected by the inclination detector just before the vehicle has come into a halt indicates that the inclination is equal to or greater than a predetermined upward inclination, then it is preferable that while the vehicle is stationary with the brake being operated and the accelerator being turned off, the control system will not allow the transmission in the drive range to be set in a neutral condition.
In a case where the inclination of the road is detected from the acceleration of the vehicle as described above, if the vehicle is brought into a halt abruptly, then an error is likely to happen in the detection of the acceleration and in the inclination detection following the acceleration detection. To solve this problem, the control system further comprises a deceleration detector (for example, the G sensor 38 described in the following section), which detects the vehicle""s deceleration while the vehicle is traveling. With this arrangement, if the vehicle""s deceleration detected by the deceleration detector just before the vehicle has come into a halt is equal to or greater than a predetermined deceleration, then it is preferable that while the vehicle is stationary with the brake being operated and the accelerator being turned off, the control system will not allow the transmission in the drive range to be set in a neutral condition notwithstanding a value detected by the inclination detector.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.